The present invention relates to machine tools in general, especially to numerically controlled profile grinding machines, and more particularly to improvements in machine tools of the type wherein a bed supports a reciprocable and indexible tool carrying device and wherein a column or an analogous supporting device is provided with a vertically reciprocable carriage for a workpiece. The following description will deal primarily with profile grinding machines but it will be appreciated that the invention can be embodied with equal or similar advantage in other types of machine tools such as lapping, nibbling and polishing machines.
The grinding of workpieces having complex profiles with a plurality of straight and/or curved parallel or mutually inclined surfaces or facets, shoulders, recesses, grooves, protuberances and/or other irregularities presents many problems to the designer of an automatic grinding machine which is utilized to treat large numbers of workpieces per unit of time. Typical examples of workpieces which must be treated in specially designed grinding machines are turbine blades, guide vanes for use in turbines and like parts which must be machined with an extremely high degree of precision and whose dimensions must match the optimum dimensions irrespective of the number of successively treated workpieces. Furthermore, it is important to ensure that such workpieces be treated economically which necessitates the grinding of a large number of different surfaces and/or facets without changing the position of the workpiece with reference to its holder.
A grinding machine wherein the workpiece is mounted on a carriage which is reciprocable along a vertical path defined by a column and wherein the supporting device for the grinding spindle is reciprocable along a horizontal path and is indexible about a vertical axis is disclosed in German Offenlegungsschrift No. 32 46 168. The machine is only capable of grinding relatively simple turbine blades and analogous workpieces because the number of different mutual positions of the workpiece and the grinding wheel is rather limited. The spindle for the grinding wheel is mounted on a first (transverse) carriage which can be indexed about a vertical axis and is reciprocable relative to a second (longitudinal) carriage which is movably mounted on the bed. The just described mounting of the spindle renders it possible to incline the grinding wheel with reference to the workpiece but the movements of the carriages are complex when the grinding wheel is to be held in inclined position while removing material from the workpiece. The versatility of the just discussed grinding machine is limited because the angle through which the axis of the grinding wheel can be changed is small; otherwise, the grinding wheel cannot be moved into actual contact with a selected portion of the workpiece. It has been found that the machine which is disclosed in the German printed application can treat only a limited number of surfaces or facets on workpieces which are as complex as parts of turbine blades or the like.
U.S. Pat. No. 4,186,529 discloses a grinding machine which is designed for the treatment of cutting tools and wherein the spindle for the grinding wheel is movable along a horizontal path together with a carriage which is mounted on a rotatable column. The work supporting device is mounted on a cross slide which is indexible about a vertical axis and is movable along a longitudinal carriage. The work clamping means is tiltable about a horizontal axis as well as about a second axis which is normal to the horizontal axis. The patented grinding machine is satisfactory for the treatment of lightweight workpieces and for the application of relatively small forces in the course of a grinding operation. This eliminates the grinding of a wide variety of workpieces which must be treated while the grinding wheel is urged against the workpiece with a substantial force, either to remove material from a hard workpiece or to remove large quantities of material per unit of time. One of the reasons for the inability of the patented grinding machine to carry out heavy-duty grinding operations is that the point of contact between the grinding wheel and the workpiece is remote from the horizontal axis about which the work clamping means is tiltable; this entails the generation of pronounced transverse forces which adversely influence the accuracy of treatment, especially when the machine is to urge the grinding wheel and the workpiece into contact with one another under the action of a large force. Accuracy of treatment is one of the most important prerequisites in connection with the grinding of a wide variety of workpieces including turbine blades, portions of guide vanes for use in turbines and the like.
A further grinding machine which is designed to treat selected parts of turbine blades and the like is disclosed in commonly owned U.S. Pat. No. 4,417,422. The construction of the patented machine is basically different from that of the aforediscussed conventional grinding machines.